Tachylite of Cleveland Dyke. 65 



resembling a spherulite in the radial arrangement of its small crystals — 

 which is outlined as a rule by tangentially disposed felspar laths. 

 There are several such patches both in the tachylite and in the ' stony 

 rock ', but they are more numerous in the latter. In all cases, 

 however, the patches in this contact variety of the Cleveland Dyke 

 are much smaller, both in general outline and in the internal 

 development of their crystalline constituents, than similar structures 

 occurring in the interior of the intrusion (e.g. in the centre of the 

 Bolam laccolite), so that the suggestion which has more than once been 

 put forward to account for thera — that they are a lateral fine-grained 

 variety of the typical rock, which has been broken up and floated 

 away by later re-heating — receives a distinct check (though not actual 

 disproof) by their scarcity at the margins and their abundance and 

 large development in the central portions of the intrusion. Moreover — 

 and it has been observed and mentioned elsewhere ' — these groups are 

 absolutely holocrystalline (which is unusual in marginal solidifications), 

 and they always consist of an internal granular mass of augite with 

 a few grains of iron-oxide and an external border of felspar laths. 



This paper deals principally with the contact varietj^ but it would 

 be incomplete without some reference to the typical rock. The general 

 structure and mineralogical composition of the Cleveland Dyke are too 

 well known to need more than the briefest description here. The 

 dyke is classed by Dr. Teall in his British Petrography as an 

 augite-andesite, " porphyritic in texture, the porphyritic character 

 being determined by the presence of tabular crystals of labradorite 

 . . . still recognizable at chilled margins." In the specimens of the 

 typical rock which wei'e examined the felspars were indeed similar to 

 those in the tachylite, but were much more numerous. The majority, 

 however, including the porphyritic individuals, must still be referred 

 to andesine rather than to labradorite, though examples of the latter 

 variety do occur. There is a greater range towards the alkaline end 

 of the felspar series here than in the tachylite. Several felspars with 

 the mean refractive index of oligoclase were seen, and some sank to 

 that of albite. These were large well-formed crystals, but, in the case 

 of the albite, of somewhat ill-defined outlines. The typical augites 

 of the Cleveland Dyke will, it is hoped, be fully described in a 

 subsequent paper ; it is sufiicient at present to say that they are 

 identical with those of the tacliylite. Iron-oxide occurs in no great 

 abundance, and even in the central parts of the intrusion it exists in 

 small incomplete or skeleton forms, so that its granular nature at the 

 margin is in no way remarkable. 



The marginal type of groundmass is described by Dr. Teall as 



^ " On the Crookdene and Belated Dykes," by Miss M. K. Heslop, M.Sc, and 

 Dr. J. A. Smythe, Q.J.G.S., February, 1910. The suggested explanation of the 

 micro-crystalline groups given in this paper has received support from subsequent 

 observations, and especially from the case under discussion. Dr. Teall very 

 kindly lent me some slides of the North of England dykes, and one was found 

 to satisfy almost perfectly the requirements of a semi-vitreous marginal solidi- 

 fication, which had been broken up into more or less angular fragments by 

 a later re-heating. In this case the fragments consisted of devitrified glass, in 

 which were embedded various elementary crystals, while they (the fragments) 

 were embedded in a similar devitrified glass with some quartz. — M. K. H. 

 DECADE V. — VOL. IX. — NO. II. 5 



