Olservations regarding Basaltic Strata and Dykes. 253 



Every competent Mineral Surveyor, or Person among those who 

 properly consult practical Sinkers, Soughers, Quarriers, &c., and 

 who collect, consult and contra'^t Specimens from the British 

 Strata, (I pretend not to speak of countries where Mineral Sur- 

 veying is unknown or unapplied) must at this day be acquainted 

 with the facts, that all the strata, without exception, though in 

 different degrees, are liable, if I may so express myself, to consi- 

 derable variations in their thicknesses, as well as in the precise 

 mineral species of which their masses are composed, in different 

 parts of their course or their dip. The same stratum, often 

 swelling into Bumps, or more commonly Ridges, and changing 

 almost completely, but more or less suddenly, many of their ex- 

 ternal and even their chemical characters, yet frequevlhj re- 

 turning to all these again, or most of them, as we proceed in 

 tracing their courses. That between the thicker and more per- 

 fect parts of very numerous strata, they are sometimes found so 

 very thin and altered, as to he with considerable difficulty re- 

 cognised ; and yet, lias each stratum, a pretty well marked place 

 in the Series, oi s7/ccessiveh/ deposited strata; such successive 

 strata eras, being marked, by the existence and extinction, of a 

 series of Organized Beings: many of these places in the series, 

 are now well ascertained and known, and by the zealous co- 

 operation of such Men as Mr. Winch, in freely and promptly 

 communicating in the pages of the Phil. Mag. a vast many more 

 may speedilv be settled, and the Geological knowledge of I3ritain 

 proportionally advanced. 



And >vith regard to Dykes or Stone Veins, there really ap- 

 pear to me, less anomalies with regard to Basalt, from Sand- 

 stone, Clay, and other mineral masses, all nhich fill Dykes in 

 vast nii7nbers, as all Colliers know, than with regard to strata 

 of these substance,-*, respectively. Without attempting the 

 task, at present an impossible one, of assigning the source or 

 origin of Mineral Matter, generally, whether composing strata 

 or filling fissures, subsequently opened in strata previously con- 

 solidated, both seeming equally beyond our comprehension ; 

 — it seems to me perfectly plain, from a very extensive series of 

 observations, that fissures were originally opened, some in nearly 

 equal degrees, so as to produce almost flat and parallel cheeks 

 or separated edges of the strata, and others of these cheeks were 

 slightly, and some very suddenly conc.ive, consistently with ^e- 

 neral or local causes of shrinking or contracting in the mass; 

 the former of these fissures, the almost parallel sided ones, com- 

 monly intersecting each other (as the doubly wedging or lenti- 

 cular ones sometimes do not), and rarely crossing, but more 

 coimnon!) ending in each other, so us to separate the mass of 



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