58 BASIC AND ULTRABASIC IGNEOUS ROCKS— BENSON. [MEMOms t i? A L.xix; 



basic rocks are intrusive into Paleozoic schists is the opinion of Iddings ('13) and as such he 

 described the gabbro-diorite complex of Salem, Mass., which was erupted during the folding 

 at the close of the Ordovician period. Clapp ('21) considers that the period of intrusion of these 

 subalkaline rocks may even have been Devonian and that gravity controlled their differentiation. 

 He believes that a long period of erosion intervened between this early intrusion and the develop- 

 ment of the possibly Carboniferous alkaline lavas and plutonic rocks. Essexitic hybrid rocks 

 were formed, however, as a result of the action of nepheline-syenite-magma on the relatively 

 cold gabbro. 



The Cortlandt series, "probably of late Paleozoic age" invades schists and limestone. 

 (Rogers '11, '11a). They prove to be more complex than Williams first supposed ('86, '87, '88). 

 The main mass is norite, with olivinic, augitic, hornblendic and quartzose varieties, exhibiting 

 gneissic banding as the result of movement under pressure during consolidation. These are 

 flanked by pyroxenite, the intrusion of which immediately preceded that of the norites. 

 Diorites invaded these, and granite forms a more isolated intrusion. Bowen ('15) suggests 

 that these may result from differentiation of a magma during which earth-movements de- 

 stroyed the evidence of gravitational control. Similar rocks appear again in Pennsylvania 

 (Bucks County norite invading limestone) and in various parts of Maryland (Iddings '13, 

 p. 394). 



Complexes in which gabbros and syenites occur together are not uncommon, examples 

 being given by Tripyramid Mountain, a marginally differentiated laccolite, in which after 

 the separation of a marginal gabbroid ring there was an access of magma bringing up the 

 inner monzonitic portion, while similarly the upper syenitic appeared last of all (Pirsson and 

 Rice '11), (or, as Bowen might hold, a chilled margin of gabbro about a gravitationally differ- 

 entiated core) and the Ascutney Mountains (Daly '03), where there are three stocks of gabbro- 

 diorite, alkali-syenite, and granite, respectively. This last is also of late Paleozoic age. Other 

 complexes occur containing granites, diorite, gabbro, and rarely peridotite and are referred 

 to various periods of Paleozoic time by Iddings ('13, pp. 374-390). The literature concerning 

 most of these the writer has not yet studied. Though nepheline-syenites are believed to belong 

 to the same series as granites, diorites, and gabbros, they do not generally occur in juxtaposition 

 with the strongly calcic numbers of the series. 3 



Mesozoic. — During Triassic times there was deposited in New Jersey and Connecticut 

 an immense thickness of sandstone and shale as terrestrial sediments, within which are inter- 

 stratified lavas, and an extensive sill obliquely truncating the stratification, while fissures were 

 filled with dikes some rising obliquely from the great sill. This is best known by its exposure 

 on the Palisades on the Hudson where it shows some gravitational differentiation (Lewis '08). 

 No folding accompanied these eruptions and intrusions, but extensive fracturing and tilting 

 of fault-blocks immediately followed (Lewis and Kummell '15). 



The States of New York, Pennsylvania, Kentucky, and Arkansas afford the most wide- 

 spread instances of the occurrence of dikes of mica-peridotites (or, better, alkaline peridotite), 

 which are entirely distinct from the mica-peridotite of the Kaltenthal or cortlandtite, but are 

 frequently porphyritic olivine-pyroxene-biotite-perofskite-bearing rocks, sometimes containing 

 melilite, more allied to limburgite, and especially alnoite. Their closest analogy is with the 

 mica-peridotites of South Africa and India, which have already been described. They invade 

 "the almost undisturbed Paleozoic strata lying to the west of the Appalachian upheavals" 

 (Kemp and Ross '07). Williams ('87a) found the rock in narrow dikes invading the Onondaga 

 salt-formation at Syracuse, and in the adjacent region about Dewitt, a further instance was 

 found to contain fragments of the crystalline country-rock (Darton and Kemp '95). Smyth 

 ('93, '02) observed melilite in these rocks. Farther south at Ithaca, similar dikes were found 

 invading the Upper Devonian Portage sandstones (Kemp '91), which are not here greatly 

 disturbed. Matson ('05) has argued (in a manner the writer does not find convincing) that 



a Attention may be called to an interesting series of albitic rocks perhaps possessing the petrographic characters of the " spilitic suite" occurring 

 near North Haven, Maine. Their associations are not known to the writer. (Iddings '13, p. 376.) A further example of the association of gabbro, 

 diorite, granite, and alkaline syenite occurring near Portsmouth has recently been described by Wandke ('22). 



