CURRENT PRE-CAMBRIAN LITERATURE 407 



of rocks, differing somewhat in age, and largely, if not almost wholly, 

 of igneous origin. Parts of this series are clearly younger than the 

 limestones ; other parts may be older than the latter formation, 

 but there is nothing as yet to prove that such is the case. A 

 probable exception to the latter statement is afforded by certain 

 laminated gneisses, of limited extent, which appear to underlie the 

 limestone, perhaps marking the base of the series. 



Many of the gneisses have heretofore been believed to be sedimen- 

 tary, and the evidence leading to the conclusion that they are largely 

 igneous may be briefly summarized: There is the negative evidence 

 of the absence of all structures pointing to sedimentary origin; the 

 uniformity of composition and structure over wide areas, with changes 

 by gradual transition; a common occurrence of massive cores, in every 

 way identical with plutonic rocks; the presence of structures in the 

 gneiss that would result from the application of pressure to igneous 

 rocks; eruptive contacts between the abundant light-colored gneiss and 

 the less common and older dark gneiss, together with widespread 

 instances of inclusions of the dark gneiss in the light; the identity 

 of the gneiss near Natural Bridge with the plutonic gabbro intrusive 

 in the limestone; eruptive contacts at a number of places of the gneisses 

 with the limestone. 



Cushing 1 describes syenite-porphyry dikes in the northern Adiron- 

 dacks. They are shown to be of pre- Cambrian age, but later than the 

 gabbros and granites of the region. The syenite-porphyries con- 

 stitute the complementary rocks to the diabases of the region, and 

 together with them form an eruptive assemblage similar to that which 

 characterized Keweenawan time in the Lake Superior region. 



Cushing 2 describes the geology of Clinton county, New York. 

 The pre-Cambrian succession, following Kemp, is as follows : (1) a 

 basal gneissic series; (2) a series of schists and gneisses, with 

 crystalline limestone ; (3) igneous rocks of the gabbro type, intrusive 

 in the first two series. All of these are overlain unconformably by 

 Paleozoic sediments. This classification is tentative, and probably 

 simpler than the one finally adopted is likely to be. 



1 Syenite-porphyry dikes in the northern Adirondacks, by P. H. CUSHING: Bull. 

 G. S. A., Vol. IX, 1898, pp. 239-256. 



2 Report on the geology of Clinton county, by H. P. Cushing: From the Fifteenth 

 Ann. Rep. of the State Geologist, in Ann. Rep. of N. Y. State Museum, 1895, 

 PP. 503-573- 



