SIXTH REPORT OF THE DIRECTOR IOXX) 25 



of this type, we are dealing with a rare variety, which is rarely 

 found except in association with nepheline-syenite. 



From this it follows that the presence or absence of melilite, 

 instead of being a mere matter of one mineral more or less in 

 the rock, determines the la Iter as belonging in the alkaline or 

 subalkaline- group, as the case may be, the peridotites belonging 

 in the latter class, the alnoites in the former. Now as this dis- 

 tinction, recognized by Iddings nearly 20 years ago, and recently 

 emphasized by Harker, is probably the most fundamental and far- 

 reaching subdivision of igneous rocks that we have, it is, of course, 

 a vital matter in the history of these dikes to determine in which 

 group they belong, and the whole matter turns upon the one mineral 

 — melilite. If the rocks contain melilite, they are alnoites and in 

 the alkaline, or, to use Harker's term, Atlantic group. On the 

 other hand, if they contain no melilite, they are peridotites and 

 belong to the subalkaline or Pacific group. As these groups are 

 not only of the first order of magnitude, but, as shown by Harker, 

 may be genetically connected with major types of crustal move- 

 ment, there is a heavy responsibility resting upon this one re- 

 grettably unstable mineral. 



Rock cities of Cattaraugus county. It is not generally known 

 that these singularly picturesque exhibits of rock decay are en- 

 tirely unique among the scenic features of this State. New York 

 is well supplied with striking scenic effects resulting from the 

 more usual procedures of natural forces ; cataracts and erosion 

 gorges, ravines and chasms, mountains of grandeur and stock- 

 aded cliffs; natural rock bridg'es and subterranean caverns. 

 These rock cities, however, particularly that situated a few 

 miles south of Olean, Cattaraugus co., are phenomena of an- 

 other order, wherein the decomposition of the rock beds under 

 meteoric ag'encies has broken and gashed the rock beds into 

 extraordinary picturesque effects. Their unusual features at- 

 tracted the special attention of the geologists at the time of the 

 original survey, 1836-42, and Professor Hall made an extended 

 reference to them, accompanied by a series of effective sketches 

 drawn by Prof. Eben N. Hosford, then his field assistant. 



These rock cities are situated high on the summits of the 

 hills and in the days of the pioneer survey were accessible only 

 with difficulty. The growth of population and facilities of inter- 

 course have now brought the most striking of these, that near 

 Olean, into easy reach of the public, by means of a trolley 

 system running thence to Bradford, Pa. over a route of rare pictur- 

 esqueness. The Olean rock city is happily under control and 

 protection from the encroachments of timber cutting and oil drill- 

 ing which had begun to attack it. 



