MEMORIAL OF H. P. GUSHING 47 



Cushing' s chief work has related to the difficult and elusive problems of 

 the Precambrian in northern New York, and by these he will be always 

 best and most widely known. He was an observer of great accuracy and 

 of keen insight, with an unusual gift for well grounded interpretations. 

 His conclusions have rarely, if ever, been seriously modified by the sub- 

 sequent study of other careful students. 



His first contributions on Clinton County, 181)4 and 1891, in addition 

 to the detailed areal geology, established the last northeasterly appearance 

 of the anorthosite and the great local abundance of the diabase dikes. 

 Aside from some additional small exposures of gneiss, the county is made 

 up of Cambrian and Ordovician strata. The dikes at Eand Hill were 

 described in great detail in 1901. 



In 1895 and 1896 Dr. James Hall was anxious to have the boundaries 

 of the Potsdam sandstone along the northern border of the Adirondacks 

 accurately mapped, and assigned Henry Cushing to the task. While 

 tracing its limits, Professor Cushing was impressed with the relations of 

 the trap dikes to these sediments. The subject of the dikes was one of 

 lively interest among the three of us working in the Adirondacks, follow- 

 ing the issue in 1893 of Bulletin 107 of the U. S. Geological Survey on 

 the trap dikes of the Lake Champlain region. Professor Cushing became 

 convinced that the Potsdam sandstone lay always over the diabase dikes, 

 which were to be observed in the neighboring Precambrian, and that 

 therefore these dikes were pre-Potsdam. This interesting observation is 

 elaborated in the second title for 1896, which quite definitely established 

 pre-Potsdam intrusives in addition to the already demonstrated post- 

 Ordovician dikes of the Champlain Valley. 



In the summer of 1898, while working along the cuts recently made 

 near Loon Lake station for the new Mohawk and Malone division of the 

 New York Central Eailroad, Professor Cushing found slabs and blocks 

 of Grenville limestone and associated strata caught up in an intrusive 

 gneissoid rock of syenitic composition, and the first of the titles for 1899 

 resulted. This observation was of great importance. C. H. Smyth had, 

 indeed, described intrusive syenites some time before from the west side 

 of the mountains, and the writer had noted the mineralogy of these 

 gneissoid augite syenitic gneisses at Ticonderoga and one or two other 

 places, but the work near Loon Lake conclusively showed that, despite its 

 gneissoid character, the syenite in the eastern mountains was intrusive 

 and that it was later than the Grenville. There remained now the inter- 

 esting time relations of the syenite series to the anorthosites. Proof posi- 

 tive of these relations was not found until the work on the Long Lake 

 Quadrangle, whose publication came in 1907. Professor Cushing in this 



