DYNAMIC METAMORPHISM OF ANORTHOSITES. 489 



the geological problems involved have become more clearly appreciated, and 

 although the survey has necessarily up to this time been largely in the nature of 

 reconnoissance, it has brought out much that is definite. Previous papers * before 

 the Society have discussed several of these points. It is evident that with a series 

 of gneisses which are in part thought to be of sedimentary origin on account of the 

 crystalline limestones involved in them, there is also present a vast amount of in- 

 trusive or plutonic rocks of the gabbro family. They constitute the high peaks 

 and ridges and include purely feldspathic aggregates or anorthosites and very basic 

 olivine-gabbros with various intermediate types, all of whose genetic relations it is 

 hoped in the future to demonstrate with chemical analyses and petrographic de- 

 scriptions. The massive varieties are comparatively simple problems, but the 

 widely prevalent gneissoid types, with the same mineralogy as the above massive 

 forms, have proved extremely puzzling. Specimens have been gradually accumu- 

 lated, however, and observations have been rec6rded, so that a practically un- 

 broken series can be established from the massive to the gneissoid, together with 

 the development of some secondary minerals of which garnet is commonest. The 

 observations are of value not alone in their local application, but in illustrating in 

 a remarkably clear way progressive dynamic metamorphism. With this object in 

 view the writer placed in serial order about twenty-five specimens which he ex- 

 hibited with comments. 



First were shown perfectly massive and coarsely crystalline anorthosites from the 

 vicinity of lake Sanford. They are dark blue or almost black aggregates of large 

 labradorite crystals and practically nothing else. Next specimens were shown in 

 which the rims of the labradorite crystals were crushed, first, with a narrow border 

 of cataclastic fragments, then with broader and broader crushed rims, until the 

 large crystals were only represented by irregular nuclei in a pulp of feldspar. The 

 extreme case involved the comminuted fragments alone. These latter badly crushed 

 varieties were called " pulp-anorthosites," and showed slight if any development 

 of foliation or of shearing. A second series of specimens illustrated this phase, 

 and the passage of the crushed anorthosites and acidic gabbros into augen-gneisses 

 and finally into thinly foliated gneisses was traced step by step. The rich develop- 

 ment of garnets in many was also shown. Starting again with massive olivine- 

 gabbro possessing an almost ophitic texture from a great ledge north of Port Henry, 

 the passage of this in the same outcrop through faintly gneissoid varieties into 

 thinly laminated types was illustrated step by step, the final product being practi- 

 cally a hornblende schist. 



All these changes were explained by crushing, flowage and shearing from dy- 

 namic processes attendant on the pre-Cambrian upheavals in the region. The least 

 crushed or sheared varieties favor the central peaks or the interior parts of the 

 larger ridges. The outer flanks are characteristically crushed or gneissoid. The 

 confident hope was expressed that by establishing such series the origin of many . 

 of the obscure gneisses could be explained. 



Acknowledgments were made to Professor James Hall, state geologist, under 

 whose direction much of the material used in illustration had been gathered. 



* Gabbros on the western Shoj-e of Lake Champlain. This Bulletin, vol. 5, p. 213. 



Crystalline Limestones, Ophioalcites and associated Schists in the Eastern Adirondacks. Idem., 

 vol. 6, p. 241. To these may be added Preliminai-y Report on the Geology of Essex county. Re- 

 port of the "New York State Geologist, 1893, p. 433. Geology of Moriah and Westport Townships. 

 Bulletin State Museum, vol. iii, p. 325. 



