eal in it is inci-easiiig. Iiowevei'. may he judged from the nutober of American sliidenla who have been and still &re pursuing it at varions Ucrman universities. What is needed in this country are well-eqiiipped pctrographii^al labo- ratories, so that those nbo are unable to avail themselves of the facilities which Europe affords may not be compelled to remain in ignorance or what is daily becoming a more und moi'e necessary part of a geologist's train- ing. An attempt to oi^nnize such a laboratory hne recently heen made at the Johns Hopkins nniversity and the encouragement which it has alreadj' received seems to abundantly jus- tify the experiment.
Heretofore microscopical petrography has been essentially a branch of mineralogy, but its future certainly lies in the far wider apiiere of geology, Tbe mere laboratory study of isolated rock -spec! mens, which has served so good a purpose in the perfecting of delicate and accurate methods, no longer possesses any signiticaucc, now that these are so tborougbly develofjed. What in Germany has been secured by jeais of patient labor may now be learned ia a com|iaraliveiy short time. Geologists have only to know and realize its application to Ibeir field of work, in order to eagerly avail them- selves of snch an important aid. Tbe use of the microscope alone will in future produce but little that is new ; but its possibilities in geol- ogj\ when intelligently employed in connection with the most detailed and careful field-work, — the necessity of wbicii has been inci'eased, not diminished, by its introduction, — cannot be easily overrated.
What paleontology has done for the foasilifer- ons deposits, this, and even more, the micro- scope must do for the crystalline rocks. The less altered forms of igneous masses have thus far been almost exclusively studied ; and, although they still have much to teach lis, it is not by their investigation that the microscope is destined to yield its greatest assistance to geology. The clianges, structural and chemi- cal, whidi go on in rocks after they are first formed, leave behind tbem more or less distinct traces which it is the special pi-ovince of the niicroscopo to follow out and interpret. Of how mut-h has already been learned regarding the alteration of setlimentary rocks near their contact with eruptive masses, the work of Rosenbuseb in the Vo^es Mountains, of Lessen in the Ilarlz. and of Hawes in New Hamp- shire, is abundant proof. The wide-spread changes which rockd subjected to regional ■netamorpliism have undergone, are far more
mplicated and difficult, but they can un-
��doubtedly be studied wilh as great success. It is by dealing with such problems as Lossen, Rcnard, and Lehmann, in Euro|)e, and Wads- wortb in this country. Lave especially pointed out. that the microscope in geology can in future render its best service. Tbe manner in which this can be accomplished is by the patient following, step by step, of unchanged rocks into their most completely altered e(]uiv- alents, and carefully comparing the condition of each constituent at every point. In this manner the succession of changes which they undergo may be as completely worked out as though we could see the process actually going on before our eyes. The alterations of olivine and enstatite to serpentine, of pyrasene to hornblende, and even the reaction of two minerals upon each other in forming a third of intermediate composition, as shown in the rim of amphibole which surrounds olivine where it is in contact with plngioclase, have nil been traced by the microscope through every stage. More recenllj' the effects of pressure exhibited by the bending and break- ing of crjstals, the disturbing of their opti- cal characters, and the local cniahing of the rock constituents, have been carefully studied. This is found almost always to be attended by the formation of new minerals, like albite, zoi- site, mica, garnet, etc., whose younger origin is only lo be proved by a microscopic investiga- tion. It is impossible to mention here a tithe of what has already been done in this direc- tion, although a beginning has hardly yet been made. What are especially to be desired are detailed studies of many small areas, where the same rock, whether eruptive or sedimentary, can be traced from its original form to its most altered state, and a comparison of the results obtained in each. This Lossen' has recently attempted for the southern liartz, and has thereby indicated what is perhaps the most promising field for microscopic work in geol- ogy. Georce H. Williams.
��THE SPANISH EARTHQUAKES.'
Thp. Spanish peninsula has been the scene of a series of earlhquakea, which, for extended duration and disasti-ous effects, surpasses any thing that has heen felt in that region in recent
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